The present invention relates to echo cancellers. More particularly, the present invention relates to a method of synthesizing the echo path of a signal in an echo canceller.
Echo cancellers are used in voice communication circuits to eliminate echoes, primarily caused by hybrid transformers which convert 2-wire circuits to 4-wire circuits. Typically, echo cancellers are based on the principle of adaptive transversal filtering. In these types of echo cancellers, the adaptive transversal filter replicates the true echo path with the opposite polarity to compensate for the imperfections of the hybrid. Generally, echo cancellers that operate on long echo tails perform poorly if all the taps of the adaptive filter are used for estimating the echo path and filtering the near end signal to generate an echo estimate.
Echo cancellers often use two adaptive transversal filters, one in the foreground and another in the background, to model the echo path. The general approach is to transfer coefficients from the background filter to the foreground filter to filter the near end signal once the echo canceller has reached some convergence state. This technique is prone to errors because double talk or a low echo return loss can result in the wrong set of coefficients being chosen for transfer to the foreground filter. To avoid these difficulties, some echo cancellers update the foreground filter only when they are sure that a double talk condition does not exist. This can result in increasing the convergence time for the echo canceller thereby degrading echo canceller performance.
Accordingly, a method of synthesizing an echo path is needed that will allow an echo canceller to quickly converge on, and accurately track, echo paths of various lengths and originating from various types of hybrids.